Continued satisfaction of ever-increasing energy demands requires tapping into the vast stores of available coal which have a high sulfur content. One significant drawback to using coal having a high sulfur content is that combustion of the coal produces sulfur dioxide (SO.sub.2) and other sulfur oxides (designated generally as "SO.sub.x "), which are highly undesirable from an environmental standpoint and require further processing prior to release into the atmosphere.
Coal pyrolysis is a well established means for desulfurizing coal. Pyrolysis entails heating the coal to a temperature sufficiently high to drive off as a gas the sulfur present in the coal without degrading the total heating value of coal or combusting the coal itself. Typically, the sulfur contained in coal is in the form of both organic and inorganic compounds. Pyritic sulfur (FeS.sub.2) is released as hydrogen sulfide (H.sub.2 S) during pyrolysis, according to the following reaction: EQU FeS.sub.2 +H.sub.2 .fwdarw.FeS+H.sub.2 S.
Additionally, the moisture in coal reacts with the ferrous sulfide (FeS) to produce mainly hydrogen sulfide, and some sulfur trioxide (SO.sub.3), according to the following reactions: EQU FeS+H.sub.2 O.fwdarw.FeO+H.sub.2 S. EQU FeS+4H.sub.2 O.fwdarw.FeSO.sub.4 +4H.sub.2 EQU FeSO.sub.4 .fwdarw.FeO+SO.sub.3
The above reactions take place at or about 400.degree. C.
The release mechanism of organic sulfur compounds from coal is not well understood; however, it is generally known that organic sulfur compounds are released in the form of hydrogen sulfide (H.sub.2 S) and carbonyl sulfide (COS), at temperatures below about 600.degree. C. It is also known that pyrolysis of coal at 500.degree. C.-600.degree. C. releases up to about 87% of the total sulfur contained in the coal. Coal pyrolysis at or about 500.degree. C. is sufficient to release a minimum of about 50% of the sulfur in the form of H.sub.2 S and COS. Subsequent combustion of the sulfur-containing gases (H.sub.2 S and COS) produced during pyrolysis results in the formation of various sulfur oxides (SO.sub.x), which must be substantially removed from the combustion gases prior to release into the atmosphere.
The removal of sulfur from coal and the removal of the gaseous sulfur by-products from coal-derived flue gas has been the goal of numerous research efforts during the last two decades. Various methods have been attempted and some have been developed to the level of commercialization. However, the cost of current desulfurization technology is still a major drawback to the increased use of coals containing a high level of sulfur and sulfur compounds.
With respect to conventional methods of wet-scrubbing flue gases or the gaseous by-products of coal pyrolysis, the capital costs and/or retrofit costs are extremely high. Dry scrubbing technologies have been developed which could conceivably offer capital cost savings over the conventional wet-scrubbing methods; however, these techniques are disadvantageous in that they do not adequately remove the amount of SO.sub.x desired without using excessive amounts of solid sorbents, due to poor sorbent utilization under flue conditions, i.e., excess oxygen. This poor sorbent utilization produces an enormous amount of scrubber solid waste, which presents its own economic and environmental disadvantages in the disposal thereof.
In addition to the above-noted problems which are due to the presence of sulfur compounds in coal, coal used for fuel may also contain on the order of 1-2% by weight nitrogen compounds which contribute up to about 70% of the oxides of nitrogen (designated generally as "NO.sub.x ") produced when the coal is combusted. These NO.sub.x are an acid rain precursor pollutant and the minimization thereof is desirable. The minimization of NO.sub.x production during coal combustion follows directly from a reduction in the amount of nitrogen present in the coal that is combusted.
What is needed is a method of desulfurizing and denitrifying coal which is efficient, cost effective, and which satisfies environmental concerns.